
Is this a first for a user who can sign a message? Or does it happen more often?
Theoretically, he can, but never did nor did we feel the need to ask for one as it wouldn't have meant much since the private keys seem to have been kept on a possibly compromised email address. There were a lot of question marks that wouldn't have been answered even with the help of a signed message in this case.
That's understandable but it would have evidenced the possibility that Legendster was telling the truth. Without it, there's no reason to believe whoever has the account is currently capable of doing it, and it could have just been part of a story.
I would have signed the message first and
then disclosed that its possible the hacker had the private key. It's a weird statement to make without backing up.
In any case, its a moot point now.